Contents

# Contents

## Idea

Given an embedding of smooth manifolds $i \colon X \hookrightarrow Y$ of codimension $n$, the Thom collapse map (Thom 54) is the continuous function from $Y$ to the n-sphere which assigns asymptotic normal distance from the submanifold, measured

1. in direction perpendicular to the submanifold, with respect to a normal framing;

2. asymptotically, regarding all points outside a tubular neighbourhood as being at infinity.

graphics grabbed from SS 19

For maximal codimension $n$, hence for 0-dimensional submanifolds, hence for configurations of points, this is alternatively known as the “electric field map” (Salvatore 01 following Segal 73, Section 1, see also Knudsen 18, p. 49) or the “scanning map” (Kallel 98).

The homotopy class of the Thom collpase map may be regarded as the Cohomotopy charge of the submanifolds, as measured in $n$-Cohomotopy-cohomology theory.

The PT collapse is a useful approximation to the would-be left inverse of the embedding of topological spaces

As such, it is is used to define pushforward of cohomology-classes along $i$ (“Umkehr maps”). It also appears as the key step in Thom's theorem.

## Definition

### Component definition in topological spaces

All topological spaces in the following are taken to be compact.

Consider $X$ and $Y$ two manifolds and

$i \colon X \hookrightarrow Y$

an embedding.

Write

• $N_i X \coloneqq i^* T Y/ T X$ for the normal bundle;

• $Th(N_i X)$ for the Thom space of the normal bundle;

• $f \colon N_i X \longrightarrow Y$ for any choice of tubular neighbourhood of $i$.

###### Definition

The collapse map (or the Pontrjagin-Thom construction) associated to $i$ and the choice of tubular neighbourhood $f$ is

$c_i \colon Y \to Y/(Y - f(N_i X)) \stackrel{\simeq}{\to} Th(N_i X) \,,$

where the first morphism is the projection onto the quotient and the second is the canonical homeomorphism to the Thom space of the normal bundle.

###### Remark

Since in the construction of remark every point of $N_i X$ is associated to a particular point of $X$, the collapse map lifts to a map

$Y \longrightarrow X_+ \wedge Th(N_i X)$

from $Y$ to the smash product of the Thom space (canonically regarded as a pointed topological space) and the topological space $X$ with a base point adjoined.

(e.g. Rudyak 98, p. 317)

###### Example

Of particular interest is the case where $Y$ in the above is a Cartesian space $\mathbb{R}^{dim X + k}$ or rather its one-point compactification, the sphere $S^{dim X + k}$. By the Whitney embedding theorem, for every $n \in \mathbb{N}$ there exists an $k \in \mathbb{N}$ such that every manifold $X$ of dimension $n$ has an embedding $X \hookrightarrow \mathbb{R}^{n+k} \to S^{n+k}$. In this case the collapse map of def. has the form

$S^{n+k} \longrightarrow Th(N_i X) \,.$

Composing this further with the canonical map $N_i X \longrightarrow E O(k) \underset{O(k)}{\times} \mathbb{R}^{k}$ to the universal vector bundle of rank $k$ yields a map

$S^{n+k} \longrightarrow M O(k)$

from to the $k$th space in the Thom spectrum $M O$. This hence defines an element in the homotopy group $\pi_{k}(M O)$ of the Thom spectrum. Thom's theorem says that all elements in the homotopy groups of $M O$ arise this way, and that they retain precisely the information of the cobordism equivalence class of manifolds $X$.

In this case the refined Thom collapse map of def. is of the form

$S^{n+k} \longrightarrow X_+ \wedge Th(N_i X) \,.$
###### Remark

The refined map in example lifts to a morphism of spectra

$\mathbb{S} \longrightarrow \Sigma_+^\infty X \wedge \Sigma^{-n-k} Th(N_i X)$

where $\mathbb{S}$ denotes the sphere spectrum and $Th(N_i X)$ now the Thom spectrum of the normal bundle.

This morphism is the unit of an adjunction which exhibits the suspension spectrum $\Sigma_+^\infty X$ as a dualizable object in the stable homotopy category, with dual object $\Sigma^{-n-k} Th(N_i X)$. See at Atiyah duality and at n-duality.

Equivalently, one may proceed as follows. For a framed manifold i.e. a manifold $M^n$ with a chosen trivialization of the normal bundle $N_i (M^n)$ in some $\mathbf{R}^{n+r}$ one has $T N_i(M^n)\cong \Sigma^r(M^n_+)$ where $M^n_+$ is the union of $M^n$ with a disjoint base point. Identify a sphere $S^{n+r}$ with a one-point compactification $\mathbf{R}^{n+r}\cup \{\infty\}$. Then the Pontrjagin-Thom construction is the map $S^{n+r}\to Th(N_i X)$ obtained by collapsing the complement of the interior of the unit disc bundle $D(N_i M^n)$ to the point corresponding to $S(N_i M^n)$ and by mapping each point of $D(N_i M^n)$ to itself. Thus to a framed manifold $M^n$ one associates the composition

$S^{n+r}\to Th(N_i X)\cong \Sigma^r M^n_+\to S^r$

and its homotopy class defines an element in $\pi_{n+r}(S^r)$.

### Abstract definition in terms of duality

The following is a more abstract description of Pontryagin-Thom collapse in the stable homotopy theory of sphere spectrum-(∞,1)-module bundles.

###### Definition

Write

$D \coloneqq (-)^\vee\circ \Sigma^\infty_+ \coloneqq L_{whe} Top \to \mathbb{S}Mod$

for the Spanier-Whitehead duality map which sends a topological space first to its suspension spectrum and then that to its dual object in the (∞,1)-category of spectra.

###### Proposition

For $X$ a compact manifold, let $X \to \mathbb{R}^n$ be an embedding and write $S^n \to X^{\nu_n}$ for the classical Pontryagin-Thom collapse map for this situation, and write

$\mathbb{S} \to X^{-T X}$

for the corresponding looping map from the sphere spectrum to the Thom spectrum of the negative tangent bundle of $X$. Then Atiyah duality produces an equivalence

$X^{- T X} \simeq D X$

which identifies the Thom spectrum with the dual object of $\Sigma^\infty_+ X$ in $\mathbb{S} Mod$ and this constitutes a commuting diagram

$\array{ && X^{- T X} \\ & \nearrow & \downarrow^{\mathrlap{\simeq}} \\ \mathbb{S} &\underset{D(X \to \ast)}{\to}& D X }$

identifying the classical Pontryagin-Thom collapse map with the abstract dual morphism construction of prop. .

More generally, for $W \hookrightarrow X$ an embedding of manifolds, then Atiyah duality identifies the Pontryagin-Thom collapse maps

$\mathbb{S} \to X^{-T X} \to W^{- T W}$

with the abstract dual morphisms

$\mathbb{S} \to D X \to D W \,.$
###### Remark

Given now $E \in CRing_\infty$ an E-∞ ring, then the dual morphism $\mathbb{S} \to D X$ induces under smash product a similar Pontryagin-Thom collapse map, but now not in sphere spectrum-(∞,1)-modules but in $E$-(∞,1)-modules.

$E \to D X \otimes_{\mathbb{S}} E \,.$

The image of this under the $E$-cohomology functor produces

$[D X \otimes_{\mathbb{S}} E, E] \to E \,.$

If now one has a Thom isomorphism ($E$-orientation) $[D X \otimes_{\mathbb{S}} E, E] \simeq [X,E]$ that identifies the cohomology of the dual object with the original cohomology, then together with produces the Umkehr map

$[X,E] \simeq [D X \otimes_{\mathbb{S}} E, E] \to E$

that pushes the $E$-cohomology of $X$ to the $E$-cohomology of the point. Analogously if instead of the terminal map $X \to \ast$ we start with a more general map $X \to Y$.

More generally a Thom isomorphism may not exists, but $[D X \otimes_{\mathbb{S}} E, E]$ may still be equivalent to a twisted cohomology-variant $[X,E]_{\chi}$ of $[X,E]$, namely to $[\Gamma_X(\chi),E]$, where $\chi \colon \Pi(X) \to E Line \hookrightarrow E Mod$ is an (flat) $E$-(∞,1)-module bundle on $X$ and and $\Gamma \simeq \underset{\to}{\lim}$ is the (∞,1)-colimit (the generalized Thom spectrum construction). In this case the above yields a twisted Umkehr map.

## Properties

### General

###### Proposition

For given $i$ all collapse maps for different choices of tubular neighbourhood $f$ are homotopic.

###### Proof

By the fact that the space of tubular neighbourhoods (see there for details) is contractible.

### Relation between cohomotopy and cobordism

For $X$ a closed smooth manifold of dimension $D$, the Pontryagin-Thom construction (e.g. Kosinski 93, IX.5) identifies the set

$SubMfd_{/bord}^{d}(X)$

of cobordism classes of closed and normally framed submanifolds $\Sigma \overset{\iota}{\hookrightarrow} X$ of dimension $d$ inside $X$ with the cohomotopy $\pi^{D-d}(X)$ of $X$ in degree $D- d$

$SubMfd_{/bord}^{d}(X) \underoverset{\simeq}{PT}{\longrightarrow} \pi^{D-d}(X) \,.$

In particular, by this bijection the canonical group structure on cobordism groups in sufficiently high codimension (essentially given by disjoint union of submanifolds) this way induces a group structure on the cohomotopy sets in sufficiently high degree.

The following terms all refer to essentially the same concept:

## References

### Pontrjagin-Thom construction

#### Pontrjagin’s construction

##### General

The Pontryagin theorem, i.e. the unstable and framed version of the Pontrjagin-Thom construction, identifying cobordism classes of normally framed submanifolds with their Cohomotopy charge in unstable Borsuk-Spanier Cohomotopy sets, is due to:

(both available in English translation in Gamkrelidze 86),

as presented more comprehensively in:

The Pontrjagin theorem must have been known to Pontrjagin at least by 1936, when he announced the computation of the second stem of homotopy groups of spheres:

• Lev Pontrjagin, Sur les transformations des sphères en sphères (pdf) in: Comptes Rendus du Congrès International des Mathématiques – Oslo 1936 (pdf)

Review:

Discussion of the early history:

##### Twisted/equivariant generalizations

The (fairly straightforward) generalization of the Pontrjagin theorem to the twisted Pontrjagin theorem, identifying twisted Cohomotopy with cobordism classes of normally twisted-framed submanifolds, is made explicit in:

A general equivariant Pontrjagin theorem – relating equivariant Cohomotopy to normal equivariant framed submanifolds – remains elusive, but on free G-manifolds it is again straightforward (and reduces to the twisted Pontrjagin theorem on the quotient space), made explicit in:

• James Cruickshank, Thm. 5.0.6, Cor. 6.0.13 in: Twisted Cobordism and its Relationship to Equivariant Homotopy Theory, 1999 (pdf, pdf)
##### In negative codimension

In negative codimension, the Cohomotopy charge map from the Pontrjagin theorem gives the May-Segal theorem, now identifying Cohomotopy cocycle spaces with configuration spaces of points:

• Peter May, The geometry of iterated loop spaces, Springer 1972 (pdf)

• Graeme Segal, Configuration-spaces and iterated loop-spaces, Invent. Math. 21 (1973), 213–221. MR 0331377 (pdf)

c Generalization of these constructions and results is due to

• Dusa McDuff, Configuration spaces of positive and negative particles, Topology Volume 14, Issue 1, March 1975, Pages 91-107 (doi:10.1016/0040-9383(75)90038-5)

• Carl-Friedrich Bödigheimer, Stable splittings of mapping spaces, Algebraic topology. Springer 1987. 174-187 (pdf, pdf)

#### Thom’s construction

Thom's theorem i.e. the unstable and oriented version of the Pontrjagin-Thom construction, identifying cobordism classes of normally oriented submanifolds with homotopy classes of maps to the universal special orthogonal Thom space $M SO(n)$, is due to:

Textbook accounts:

#### Lashof’s construction

The joint generalization of Pontryagin 38a, 55 (framing structure) and Thom 54 (orientation structure) to any family of tangential structures (“(B,f)-structure”) is first made explicit in

and the general statement that has come to be known as Pontryagin-Thom isomorphism (identifying the stable cobordism classes of normally (B,f)-structure submanifolds with homotopy classes of maps to the Thom spectrum Mf) is Lashof 63, Theorem C.

Textbook accounts:

Lecture notes:

• John Francis, Topology of manifolds course notes (2010) (web), Lecture 3: Thom’s theorem (pdf), Lecture 4 Transversality (notes by I. Bobkova) (pdf)

• Cary Malkiewich, Section 3 of: Unoriented cobordism and $M O$, 2011 (pdf)

• Tom Weston, Part I of An introduction to cobordism theory (pdf)

The general abstract formulation in stable homotopy theory is in sketched in section 9 of

and is in section 10 of

with an emphases on parameterized spectra.

### Stratified versions

Instead of relating (structured) cobordisms with homs into Thom spectra, one may also relate stratified cobordisms with homs into cell complexes. A version of this relation is spelled out in Section VII.4 of:

• Sandro Buonchristiano, Colin Rourke, and Brian Sanderson. A geometric approach to homology theory. Vol. 18. Cambridge University Press, 1976.

The stratified Pontrjagin-Thom construction may also be considered in higher categorical terms, where it relates functors between weak computads to manifold diagrams. This is discussed at:

Last revised on September 24, 2022 at 13:47:03. See the history of this page for a list of all contributions to it.